1. Field of the Invention
The present invention relates to a method of improving the combustion in a high-temperature industrial kiln and to a device for improving the combustion in such a kiln.
2. Related Art
It is known that high-temperature industrial processes using as energy supply fuels having a not insignificant nitrogen content, such as coal or petroleum coke, generate substantial emissions of nitrogen oxides (NOx). NOx is the collective term to denote all nitrogen oxides, particularly nitrogen monoxide (NO) and nitrogen dioxide (NO2). Two types of NOx may mainly be distinguished, depending on the mechanism of their formation: fuel NOx and thermal NOx. Fuel NOx results from the oxidation of the nitrogen compounds in the fuel. Thermal NOx, which corresponds to oxidation of atmospheric nitrogen by the combustion oxygen, depends mainly on three variables:                the oxygen concentration in the high-temperature zones of the flame (>1200° C.);        the residence time of the oxygen in these zones; and most particularly        the temperature in these zones.        
NOx is toxic to plants and nitrogen dioxide in particular may trigger respiratory difficulties in humans. NOx is also one of the main precursors in ozone formation. In addition, NOx emissions contribute to acidification and eutrophization of the soil.
The problem of Nox emissions arises in all industries that use high-temperature processes. One of the industries particularly concerned is that of cement manufacture, in which the manufacturing processes are subject to stricter standards relating to nitrogen oxide (NOx) emissions. In these cement production processes the formation of NOx from fuel is due to the use of fuels having a not insignificant nitrogen content, for which the oxidation of the nitrogen-containing compounds of the fuel results in the formation of NO. This mechanism takes place both in the burner of the rotary kiln, upon ignition of the fuel, and in the precalciner when there is one. Thermal NOx for its part is inevitable in the combustion zone of the rotary kiln, because of the need for a temperature high enough for what is called the clinkering reaction carried out on the raw feed (1450° C., thus accelerating the oxidation of atmospheric nitrogen.
The current techniques for reducing the NOx emissions may be put into two categories: primary techniques limiting NOx formation during combustion and secondary techniques based on treatment of the flue gases in order to remove the NOx created upstream.
To allow the formation of NOx to be effectively reduced in cement production processes, any primary technique must limit the formation of both fuel NOx and thermal NOx. Among the main primary measures taken, the following may be listed: low-NOx burners, which optimize the mixing of the fuel and of the various oxidizer injections so as to limit mainly the formation of thermal NOx by a local combustion staging effect. This method reaches its limits in the flame instabilities that are generated when the primary air is reduced below acceptable limits (˜10% of the stoichiometric air needed). The achievable reductions are thus around 30%;                flame cooling by water injection, which has the objective of reducing the thermal NOx by lowering the temperature peaks in the flame. Thus, NOx reductions up to 50% may be achieved, but this method significantly reduces the combustion efficiency and proves to be the cause of problems in the operation of the kiln; and        staging of the combustion between the rotary kiln and the precalciner when there is one, which makes it possible to reduce the NOx at high temperature at the outlet of the rotary kiln and to subsequently complete the combustion downstream, in the precalciner and the preheating unit. NOx reduction levels up to 50% are claimed, but these systems are expensive in terms of investment cost because of substantial modifications to the installation that they require. Also mentioned are many problems of excess CO formation, which prevent regular NOx reduction levels from being obtained.        
At the present time none of these primary techniques is capable of reducing NOx emissions sufficiently thereby obliging cement producers to use costly secondary methods in order to meet the standards in force.
The secondary measures taken are conventional ones: these involve catalytic or non-catalytic NOx reduction processes (SNCR=selective non-catalytic reduction; SCR=selective catalytic reduction) based on injecting ammonia or urea into the flue gases in order to reduce the NO to N2. Larger NOx reductions are therefore possible, but for significantly higher investment and operating costs. Furthermore, these techniques require very precise temperature ranges and any deviation may then result in the emission of unreacted ammonia in the flue gases, which may then oxidize to NOx.
Apart from reducing NOx emissions, another major worry of cement manufacturers is how to achieve satisfactory efficiency and quality. Techniques employing the use of oxygen or oxygen-enriched gas have been developed. They are mainly designed to increase the production or quality of the product by enabling the temperature in the clinkering zone to be increased. Consequently, these techniques generally result in an increase in the NOx emission levels compared with operation without added oxygen, or at best said levels remaining the same.
Document U.S. Pat. No. 3,397,256 describes the use of an oxy-fuel burner placed between the charge and the main burner, with the effect that there is a significant increase in the temperature in this zone and therefore inevitably in the amount of NOx emitted.
Document U.S. Pat. No. 5,572,938 discloses the injection of oxygen into the primary air, via the main burner, for the purpose of improving the heat transfer to the charge and the production. No details are given as to the method of injection that limits the formation of fuel NOx. Oxygen injection is also proposed exclusively in the bottom part of the rotary kiln, along the charge, so as to stage the combustion. This specific positioning makes it possible to maintain oxidizing conditions above the charge and to transfer more energy thereto, but does not allow suitable mixing with all of the unburned materials.
Document U.S. Pat. No. 5,580,237 describes an injector for optimizing the injection of oxygen in the burner, for the purpose of flame stabilization. The amount of NOx emitted is maintained or slightly reduced.
U.S. Pat. No. 6,309,210 of the Applicant Company teaches the oxygen enrichment of primary, secondary and tertiary air so as to improve the cooling capability of the clinker and to improve the combustion in general. General dilution of the oxygen in all the combustion gases goes counter to the principles of reducing the amount of NOx emitted.